This fixes the following warning with GCC:
    libavcodec/pthread.c:35:0: warning: _GNU_SOURCE redefined [enabled by default]
    <command-line>::0: note: this is the location of the previous definition

The reason of the presence of this flag is:
    % pkg-config --cflags sdl
    -D_GNU_SOURCE=1 -D_REENTRANT -I/usr/include/SDL